What is Non ST elevation myocardial infarction (NSTEMI)?

Definition of Non-ST Elevation Myocardial Infarction (NSTEMI)

NSTEMI is defined as cardiomyocyte necrosis occurring in the clinical context of acute myocardial ischemia, characterized by elevated cardiac biomarkers (troponin above the 99th percentile upper reference limit with a rise and/or fall pattern) in the absence of persistent ST-segment elevation on ECG. 1

Diagnostic Criteria

NSTEMI requires the simultaneous presence of three essential components:

• Elevated cardiac troponin levels (preferably high-sensitivity cardiac troponin T or I) with at least one value above the 99th percentile of the upper reference limit, demonstrating a rise and/or fall pattern 1
• Appropriate clinical context consistent with acute myocardial ischemia, typically manifesting as chest pain, pressure, tightness, or burning, though chest pain-equivalent symptoms (dyspnea, epigastric pain, arm pain) may occur 1
• Absence of persistent ST-segment elevation on 12-lead ECG (>20 minutes duration) 1

ECG Characteristics

The ECG in NSTEMI may demonstrate several patterns, but persistent ST-elevation is notably absent:

• Transient ST-segment depression 1
• T-wave inversions (may be prominent) 1
• Flat T-waves or pseudo-normalization of T-waves 1
• The ECG may be entirely normal despite myocardial necrosis 1

Pathophysiological Distinction

NSTEMI represents a specific point on the acute coronary syndrome spectrum:

• Type 1 MI (most common NSTEMI): Results from atherosclerotic plaque rupture, ulceration, fissure, or erosion with resulting intraluminal thrombus causing decreased myocardial blood flow and/or distal embolization leading to myocardial necrosis 1
• Type 2 MI (demand NSTEMI): Occurs when conditions other than coronary plaque instability cause an imbalance between myocardial oxygen supply and demand (hypotension, hypertension, tachyarrhythmias, bradyarrhythmias, anemia, hypoxemia, coronary spasm) 1, 2

Critical Distinction from Unstable Angina

The sole differentiating factor between NSTEMI and unstable angina is the presence of detectable cardiac biomarkers of necrosis. 1, 3

• Both conditions share similar pathogenesis and clinical presentations 1
• Both vary only in severity, with NSTEMI indicating ischemia severe enough to cause detectable myocardial damage 1
• With increasingly sensitive troponin assays, biomarker-negative ACS (unstable angina) is becoming progressively rarer 1

Common Diagnostic Pitfalls

Abnormalities on ECG and elevated troponins in isolation are insufficient to diagnose NSTEMI—they must be interpreted within the appropriate clinical context. 1

• Avoid missing true posterior MI, which may present without anterior ST-elevation but represents a STEMI-equivalent requiring immediate reperfusion 1, 4
• Recognize that 5-10% of NSTEMI cases may have non-obstructive coronary atherosclerosis or no angiographic evidence of coronary artery disease, particularly in women 1
• Be vigilant for atypical presentations in women, elderly patients, and diabetics who may present without chest pain 4